Chapter 19.3, Problem 19.3SC

Exercise19.3Watches with numerals that glow in the dark” formerly were made by including radioactive radium in the paint used to letter the watch faces. Assume that to make the numeral 3 on a given watch, a sample of paint containing $\text{8}.0\times \text{1}{0}^{-\text{7}}$ mole of $\text{R}\text{a}$ was used. This watch was then put in a drawer and forgotten. Many years later someone finds the watch and wishes to know when it was made. Analyzing the paint, this person finds $1.0\times \text{1}{0}^{-\text{7}}$ moles of $\text{R}\text{a}$ in the numeral 3. Flow much time elapsed between the making of the watch and the finding of the watch?

Watches and clocks with radium dials.